Mobile telephone cellular communications systems divide geographic areas into cells, each cell being served by a cell site having a transmitter and receiver.
When a user in a car (the "mobile unit") desires to make a call, the mobile telephone unit scans the set-up channels, selects the strongest and locks on for a certain time. Each cell site is assigned a different set-up channel, so locking on to the strongest usually means selecting the nearest cell site.
A call request sent from the mobile unit is received by a cell site which typically selects a directive antenna for the communications channel. At the same time, the cell site also sends a request for a voice channel to the Mobile Telephone Switching Office (MTSO) via a high-speed data link. The MTSO selects an appropriate voice channel for the call and the cell site links the channel with the directive antenna to the mobile unit. The MTSO also connects the wire-line party through the telephone company central office.
A call from a land-line party to a mobile unit goes first to the telephone company central office which recognizes that the number is for a mobile unit and forwards the call to the MTSO. The MTSO sends a paging message to certain cell sites based on the mobile unit number and a search algorithm. Each cell site transmits the page on its own set-up channel. The mobile unit recognizes its own identification on a strong set-up channel, locks onto it, and responds to the cell site instruction to tune to an assigned voice channel.
Communications with mobile units passing from one cell to another are maintained by handoff schemes whereby cell sites transfer the communications between each other.
A core concept of cellular mobile communications systems is the reuse of frequency channels in order to increase the efficiency of use of a limited allocated spectrum. In a frequency reuse system, users in different geographic locations (different cells) simultaneously use the same frequency channel. This increases the spectrum use efficiency, but also may produce serious interference problems. Interference due to the common use of the same channel is called cochannel interference.
The minimum distance between cells using the same channel depends on factors such as the number of cochannel cells in the vicinity, the terrain, antenna heights, and transmission power at each cell site. To maximize spectrum utilization efficiency, it is necessary to minimize the distance between cells using the same channel while maintaining a sufficiently low level of cochannel interference. A comprehensive discussion of minimum distance and cochannel interference can be found in Mobile Cellular Telecommunications Systems, Chapter 2, (McGraw-Hill 1989) by William C. Y. Lee.
It is a difficult undertaking to accurately determine the areas of serious cochannel interference in a mobile communications system. A first test, simply described, involves transmitting on one channel at night while the mobile unit travels in one of the cochannel cells. A field-strength recorder in the mobile unit monitors changes in signal compared with the condition of no cochannel transmission. The test must be repeated as the mobile unit travels in every cochannel cell. A channel scanning receiver in the mobile unit records the signal level on the no-cochannel condition on one channel, the interference level on another channel, and a third non-utilized channel. By comparing the signal levels in the channels, the sufficiency of coverage may be estimated and the areas of cochannel interference spotted.
A second test involves, in a first approach, simultaneously using mobile units travelling in each cochannel cell and monitoring the signal levels. Since it is difficult to use multiple mobile units simultaneously, an alternative approach is to use a single moving mobile unit in one of the cochannel cells at a time and to record the signal strength at every other cochannel cell site. By determining the highest and lowest average signal strengths, the carrier-to-interference ratio received at a particular cell may be estimated. For details of these tests, see Mobile Cellular Telecommunications Svstems, Chapter 6, (McGraw-Hill 1989) by William C. Y. Lee.
Real-time cochannel interference measurements at the mobile transceivers are difficult to achieve in practice. Simply stated, this is because the sampling delay time must be sufficiently small such that both the amplitudes of the signal and the interference have not changed appreciably in the interim. A description of this problem can be found on pages 183-184 of Lee's book supra.
The determination of cochannel interference in practice is clearly difficult and the estimation of cochannel interference is crucial to the design of cellular mobile communications systems. Thus, it would be a significant advance in the art to have a frequency generator which can generate a realistic sequence of frequencies to simulate cochannel interference in a mobile communications system.
There are also other kinds of interference in mobile communications system, termed "noncochannel interference." "Adjacent-channel interference" includes next-channel (the channel next to the operation channel) and neighboring-channel (more than one channel away for the operating channel) interference. Next-channel interference must originate at other cell sites. This is because any channel combiner at the cell site must combine the selected channels. Therefore, next-channel interference will arrive at the mobile unit from other cell sites if the system is not designed properly. Also, a mobile unit initiating a call on a control channel in a cell may cause interference with the next control channel at another cell site.
Neighboring-channel interference typically results when all the channels are simultaneously transmitting at one cell site. Avoidance of interference problems requires a sufficient amount of band isolation between channels.
Further, there is a type of adjacent-channel interference that is unique to mobile communications systems. Since many mobile units are in motion simultaneously, their relative positions change with time. Some mobile units are close to the cell site and some are not. The close-in mobile units have strong signals which cause adjacent-channel interference. This type of interference is called near-end/far-end interference. There also can be interference between different mobile communications systems. That is, if a mobile unit in one system is closer to a cell site of another system while a call is initiated through the first system, then adjacent channel interference may be produced.
Finally, because some UHF television channels overlap cellular mobile channels, there may be UHF TV signal interference with mobile cellular signals. There may also be interference from long-distance telephone communications.
The communications environment itself contains naturally occurring non-linear junctions (such as buildings and other structures) which act upon transmitted signals to generate cross-products or intermodulation products between simultaneously transmitted signals. These products generate signals having frequencies the same or nearly the same as those carrying desired information and can thus be sources of interference.
Operable designs of mobile communications systems must consider the various kinds of interference described above. Many methods and devices to reduce interference are discussed in Mobile Cellular Telecommunications Svstems, Chapter 7, (McGraw-Hill 1989) by William C. Y. Lee. The choice of an optimum system design requires knowledge of the effects of interference. This knowledge can be gained by field testing through reproduction of the interference environment or by computer simulation. Computer simulation has the disadvantage that it does not allow a physical test of all the equipment and personnel. Thus, the best test of a mobile communications system is a subjective voice-quality monitoring for a given carrier-to-noise ratio. Therefore, there is a distinct need for an interference simulating device to subjectively test the voice quality of mobile communications systems and to rapidly and accurately identify which of a multitude of frequencies are causing the interference.
Prior art interference frequency generators include U.S. Pat. No. 4,317,214 to Attinello which injects interference signals into a receiver to simulate environmental interference effects. The types of interference injected include white noise, pseudo-random pulses, sawtooth or comb signals and other types of signal distortion.
U.S. Pat. No. 3,806,809 to Firman discloses a frequency generator simulating intermodulation interference. The generator produces, mixes, and non-linearly amplifies harmonic and beat frequencies of the carrier signal to produce a spectrally-expanded composite signal. This composite signal can then be analyzed for the main interference component affecting the carrier signal.
The prior art described above can not, however, produce frequencies with spacing of less than 150 Khz from a single multiplier. Such close frequency spacing is necessary to adequately and realistically simulate the signal transmissions from mobile communication cell sites.